Field of the Invention
The present invention relates to an image pickup apparatus and a control method.
Description of the Related Art
There has been proposed for an image pickup apparatus such as a single-lens reflex camera and the like a plurality of focus adjusting modes. One of the focus adjusting modes allows light flux from an object passed through exit pupil regions different from each other in an imaging optical system to form an image on a pair of line sensors and calculates a defocus amount of the imaging optical system from the phase difference between a pair of image signals obtained by the pair of line sensors to adjust its focus. At this time, the line sensor for detecting the image signal is a provided sensor dedicated to focus detection (sensor dedicated for focus detecting).
Also, there is another focus adjusting mode that uses an imaging element. In this type of the focus adjusting mode, an imaging (capturing) is performed by pupil-dividing imaging pixels in the imaging element with a microlens and receiving an optical axis with a plurality of focus detecting pixels, while driving the lens by calculating the defocus amount based on the pair of image signals output by the imaging.
Japanese Patent Laid-Open No. 2001-083407 discloses an image pickup apparatus having an imaging element in which a microlens for dividing an object image into a pair of images is provided in two photoelectric conversion units. Also, Japanese Patent Laid-Open No. 2009-109631 discloses an image pickup apparatus for calculating the defocus amount based on phase difference between the pair of image signals obtained in continuous shooting. Also, Japanese Patent Laid-Open No. 2008-129174 proposes an image pickup apparatus for suitably using each focus adjusting mode included in the plurality of focus adjusting modes.
In continuous shooting with automatic focus adjustment (AF), either continuous shooting consisting of the focus adjusting mode with the sensor dedicated to focus detecting or that consisting of the focus adjusting mode with the imaging element has been performed, because a continuous shooting speed is reduced by operating both the sensor dedicated to focus detection and the imaging element at all times in the continuous shooting.
In the continuous shooting using the focus adjusting mode with the sensor dedicated to focus detection, the sensor accumulates an image signal during exposure, in other words, the time from a photographing of an image to the photographing of the next image, and the defocus amount is calculated to drive the lens. This continuous shooting requires time to accumulate the image signal in the sensor, and the accumulation time is determined depending on brightness. For example, the time is short when it is bright, and the time is long when it is dark. Generally, while having the same brightness, the more capable an imaging element is of directly receiving the light adding the pixels, the shorter an accumulation time.
Also, when temperature is significantly outside of normal temperature, it is difficult to perform corrections to maintain the distance relationship between the sensor and the imaging element because the temperature causes changes of the optical path length from the photographing lens to the sensor dedicated to focus detection or the length of a flange back of the imaging element. Thus, in this case, the focus adjusting mode with the sensor dedicated to focus detection often performs the focus detecting with larger error compared to that with the imaging element.
In contrast, in the continuous shooting by the focus adjusting mode using the imaging element, the defocus amount is calculated by using the exposure to photograph a photographing image to drive the lens as disclosed in Japanese Patent Laid-Open No. 2009-109631. In the continuous shooting by the focus adjusting mode using the imaging element, occasionally, the image processing of the photographing image or the like is performed in parallel with the processing for calculating the defocus. In other words, both processing are performed by one CPU, and it may take a long time to calculate the defocus amount. Also, since this mode uses the imaging element, a reading direction is fixed. For example, the precision for the focus detection is reduced when the reading direction is horizontal because the pair of image signals is acquired in a horizontal direction. In contrast, the sensor dedicated to focus detecting can be arranged to detect both a vertical line and a horizontal line because it has a degree of freedom with respect to the arrangement in the directions of the line sensors.
Also, in the focus adjusting mode with the imaging element, distortion of the image is caused more easily compared to the focus adjusting mode with the sensor, and cannot detect the focus when the defocus amount is large. Thus, the continuous shooting speed may be reduced occasionally in each of focus adjusting modes. Also, there is a condition that the focus adjusting modes cannot be applied together, and the continuous shooting speed is reduced by using both of the focus adjusting modes under this condition.